The disclosure relates to spray deposition/coating. More particularly, the disclosure relates to cold spray nozzles.
The cold spray process is an important technology in the areas of additive manufacturing, repair, and functional coatings. It is characterized by “layer by layer” deposition build-up of material at a substrate surface by high speed impact of solid particles. The basic cold spray process involves the flow of a pressurized gas (e.g., nitrogen, helium, air, argon, hydrogen, and the like) through a gas heater (e.g., heating to between room temperature and 1000° C. effective to impart desired plasticity to the powder). Powder is injected into the heated gas stream and the powder-gas mixture is then accelerated through a de-laval type nozzle (e.g. converging-diverging) and then discharged at a substrate resulting in deposition and consolidation of the material.
Cold spray typically does not involve melting of the powder feedstock. Rather, the heating of the carrier gas combined with the high velocity (and thus kinetic energy) of particles produces highly plastic behavior of the particles on impact with the substrate and then with already-sprayed material (e.g., prior layers of the cold spray). Depending on the particular coating material and end use, artifacts of cold spray may have various benefits. These artifacts include: work hardening during impact; beneficial compressive residual stresses in the spray deposits; unique microstructures (nano-grained, multiphase materials, etc.); retention of feedstock microstructure (unlike high temperature deposition techniques (high velocity oxy-fuel (HVOF), plasma spray, etc.); and, despite the lack of melting, near 100% density if desired.
Exemplary apparatus and nozzles therefor are disclosed in United States Patent Application Publications 20160221014 A1 (the '014 publication of Nardi; Aaron T. et al., Aug. 4, 2016) and 20160222520 A1 (the '520 publication of Kennedy; Matthew B. et al., Aug. 4, 2016), the disclosures of which publications are incorporated by reference in their entireties herein as if set forth at length.
The nozzle can be made from many materials depending on the powder material being deposited, but often is cemented carbide for robustness/durability. Although the cold spray process has received considerable attention, it does however exhibit a critical drawback. Quite often, the powder material quickly clogs the nozzle resulting in fouling, poor deposits, and disruption of the process. X. Wang, B. Zhang, J. Lv, and S. Yin, “Investigation on the Clogging Behavior and Additional Wall Cooling for the Axial-Injection Cold Spray Nozzle”, Journal of Thermal Spray Technology, Feb. 25, 2015, Vol. 24 (4), pp. 696-701, Springer Science+Business Media LLC, New York, N.Y. When using typical spray powders (e.g., nickel, copper, titanium and their respective alloys) clogging can occur in as little as a few minutes, but is highly dependent on the spray process conditions and gases being used. For instance, using helium at high pressure and with high gas temperatures produces the highest particle velocities and, for many important powders, the best properties, but these instances result in the highest likelihood for clogging. The nozzle clogging may relate to adhesion mechanisms in tribological applications. Cobalt is the soft phase in the WC—Co nozzle and it is likely that powders adsorb on the Co phase during spraying.
Such clogging results in lost time and additional material cost due to frequent nozzle repair and replacement. It is due partly to this issue that extensive, long-duration industrial cold-spray processes have not yet been established.